In the past, multi-stage amplifiers have been used to achieve the high optical signal gain needed to amplify signals propagating in optical fibers. In a single stage optical amplifier, the overall optical signal gain is limited by the buildup of amplified spontaneous emission which clamps the maximum population inversion that can be achieved in the active dopant in the fiber core (typically Er, Yb, Er/Yb, Nd, Tm). The buildup of amplified spontaneous emission can be suppressed by placing several elements between the amplification stages. These include optical isolators to eliminate backward propagating amplified spontaneous emission, narrow-band spectral filters to eliminate most of the broad-band amplified spontaneous emission, and polarizers which block one-half of the unpolarized amplified spontaneous emission. In addition, for amplification of optical pulses, optical switches or “gates” can be placed between the stages. These gates are operated so as to have a high transmission state when a pulse is propagating through the amplifier, and to have a low transmission state in the time between pulses. This prevents the buildup of amplified spontaneous emission power in the time between pulses, resulting in a substantial decrease in the average amplified spontaneous emission power generated by the amplifier.
Multi-stage, core-pumped (where the pump light is directly injected into the fiber core using a fused fiber wavelength division multiplexer, as described in U.S. Pat. No. 4,938,556 to Digonnet et al., wavelength-division multiplexer-optical amplifiers) can be constructed using a separate pump source (typically a 980 nm laser diode pigtailed with a single mode fiber pumping an Er-doped fiber core) to pump each stage. Since the fiber-coupled pump diode is one of the most expensive components in a fiber amplifier, in order to decrease the overall amplifier cost and complexity, it is desirable to use a single pump diode and split the pump signal into multiple single mode fibers using a single mode fiber coupler/splitter. Each of these multiple pump fibers can then be used to supply pump power to different amplifier stages. Alternately, remnant pump light that is unabsorbed after the first amplification stage can be coupled out of the fiber core using a wavelength division multiplexer fiber coupler, allowed to bypass the isolator or other inter-stage optical elements, and then re-injected using a second wavelength-division multiplexer into another amplification stage.
The drawback of the multi-stage amplifier configuration pumped by a single mode fiber pigtailed pump diode is that the pump power available from these sources is typically limited to <400 mW. Such low available pump power limits the maximum gain and output power that can be achieved in a multi-stage amplifier pumped by a single mode fiber pigtailed pump.
To increase the amplifier output power, double cladding fibers with active doped cores can be used, as described in U.S. Pat. No. 4,815,079 to Snitzer et al. The double cladding fiber consists of a low refractive index outer cladding surrounding a higher index inner cladding (typically 50-500 μm diameter) for guiding the pump light, and a yet higher index core (typically 5-25 μm diameter) for guiding the signal light. Since inner cladding of the double cladding fibers constitutes a large multimode waveguide for the pump light, broad area laser diodes with wide emission region (50-200 μm), allowing high output power (5-10 W), can be used as the pump source.
The prior art includes many ways of injecting pump light into the double cladding fiber. These include (1) optical couplers based on optical contact between two fibers (described in U.S. Pat. No. 4,815,079 to Snitzer et al.), (2) micro-prisms (described in U.S. Pat. No. 4,815,079 to Snitzer et al.), (3) tapered fiber bundles (described in U.S. Pat. No. 5,864,644 to DiGiovanni et al.), (4) v-groove side-pumping (described in U.S. Pat. No. 5,854,865 to Goldberg), (5) fused tapered fiber pump fibers (described in U.S. Pat. No. 5,999,673 to Valentin et al.), (6) multiple-fiber fiber cables (described in U.S. Pat. No. 6,826,335 to Grudinin et. al.), and (7) 3-fiber double cladding fiber couplers (described in U.S. Pat. No. 6,434,295 to MacCormack et al.). These methods are designed to achieve near-complete transfer of the pump power from the pump diode into the doped double cladding fiber. To make a multi-stage amplifier, references (2)-(6) require use of multiple separate pump couplers, each pumping a corresponding gain stage of the multi-stage amplifier. This increases the overall amplifier cost and complexity. Methods described in reference (7) can be used to make a multi-stage amplifier if more than one double cladding fiber is in contact with the pump fiber. However, this method requires the fabrication of a complex multi-fiber cable. Reference (7) uses a 3-fiber coupler consisting of a multimode pump fiber and two double cladding fibers. The cladding of all fibers are fused together to allow transfer of the pump light from the multimode pump fiber into the claddings of the double cladding fibers. Equal amounts of pump power are coupled into the two pump fibers and no method is described for coupling different amounts of power into the double cladding fibers. Other deficiencies are that it uses 3 fibers instead of 2 fibers, and it does not describe how to construct a multi-stage amplifier using a single coupler.